The field of the invention
[0001] Most of the energy of the world is produced by means of oil, coal, natural gas or
nuclear power. All these production methods have their specific problems as far as,
for example, availability and friendliness to environment are concerned. As far as
the environment is concerned, especially oil and coal cause pollution when they are
combusted. The problem with nuclear power is, at least, storage of used fuel.
[0002] Especially because of the environmental problems, new energy sources, more environmentally
friendly and, for example, having a better efficiency than the above-mentioned energy
sources, have been developed. Solid oxide cells operate via a chemical reaction in
an environmentally friendly process and are very promising future energy conversion
devices. The intermittency of renewable energy sources has introduced challenges for
the electrical grid stability, calling for increased demand and supply side flexibility
and new energy storage and conversion technologies. Electrolysis can serve these purposes,
providing a method to generate clean hydrogen or hydrocarbons using renewable electricity
as driving force. Among all electrolysis technologies, solid oxide electrolysis (SOEC)
provides the highest efficiency.
The state of the art
[0003] Solid oxide cell (SOC), as presented in fig 1, comprises a fuel side 100 and an oxygen
rich side 102 and an electrolyte material 104 between them. In solid oxide fuel cells
(SOFCs) oxygen 106 is fed to the oxygen rich side 102 and it is reduced to a negative
oxygen ion by receiving electrons from the oxygen rich side. The negative oxygen ion
goes through the electrolyte material 104 to the fuel side 100 where it reacts with
fuel 108 producing water and also typically carbon dioxide (CO2). Fuel side 100 and
oxygen rich side 102 are connected through an external electric circuit 111 comprising
a load 110 for the fuel cell operating mode withdrawing electrical energy out of the
system. The fuel cells also produce heat to the reactant exhaust streams. In electrolysis
operating mode, current flow is reversed and the solid oxide cells act as a load to
which electricity is supplied. Depending on operating conditions, the cell operation
can be endothermic, exothermic or thermoneutral.
[0004] Fuel cell reactions in the case of methane, carbon monoxide and hydrogen fuel are
shown below:
Fuel side:
CH4 + H2O = CO + 3H2
CO + H2O = CO2 + H2
H2 + O2- = H2O + 2e-
Oxygen rich side:
O2 + 4e- = 2O2-
Net reactions:
CH4 + 2O2 = CO2 + 2H2O
CO + 1/2O2 = CO2
H2 + 1/2O2 = H2O
[0005] In electrolysis operating mode (solid oxide electrolysis cells, (SOEC)) the reaction
is reversed, i.e. electrical energy from a source 110 is supplied to the cell where
water and often also carbon dioxide are reduced in the fuel side forming oxygen ions,
which move through the electrolyte material to the oxygen rich side where oxidation
reaction takes place. It is possible to use the same solid oxide cell in both SOFC
and SOEC modes.
[0006] Solid oxide electrolyzer cells operate at temperatures which allow high temperature
electrolysis reaction to take place, said temperatures being typically between 500
- 1000 °C, but even over 1000 °C temperatures may be useful. These operating temperatures
are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen
and oxygen gases. The reactions for one mole of water are shown below:
Fuel side:
H2O + 2e- ---> 2 H2 + O2-
Oxygen rich side:
O2- ---> 1/2O2 + 2e-
Net Reaction:
H2O ---> H2 + 1/2O2.
[0007] In case of co-electrolysis, a carbonaceous species is supplied to the cell in addition
to steam, typically in proportions favorable for subsequent refining of the result
gas according to e.g. the Fischer-Tropsch process. Carbon dioxide can be directly
reduced to carbon monoxide or can interact with hydrogen through the water-gas shift
reaction to form carbon monoxide and steam.
[0008] In Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyzer (SOE) stacks where the
flow direction of the fuel side gas relative to the oxygen rich side gas internally
in each cell as well as the flow directions of the gases between adjacent cells, are
combined through different cell layers of the stack. Further, the fuel side gas or
the oxygen rich side gas or both can pass through more than one cell before it is
exhausted and a plurality of gas streams can be split or merged after passing a primary
cell and before passing a secondary cell. These combinations serve to increase the
current density and minimize the thermal gradients across the cells and the whole
stack.
[0009] The high operating temperature in the SOC cells and system introduce material related
challenges with respect to thermomechanical forces, material properties, chemical
stability and uniformity of operating conditions. These aspects place practical constraints
on feasible SOC cell, stack and module sizes. Scaling the technology for large installations,
typical to SOEC application, will thus primarily rely on multiplication of cells,
stacks and SOC modules. Minimizing the cost of each multiplying unit at all levels
is thus crucial for reducing the overall cost.
[0010] A SOC module comprises tens up to hundreds of SOC stacks, support structures, thermal
insulation, reactant conveying and distribution structures, instrumentation as well
as electrical and reactant interfacing towards the application or other modules. As
high temperature interfaces are costly, space-consuming and may constitute an ignition
source, it is also beneficial to include heat exchanging within the module to lower
the temperature of the reactant interfaces. Furthermore, the SOC module needs internal
or external means facilitate safe start-up and shutdown.
[0011] To avoid oxidation of the fuel side electrode, a sufficient portion of hydrogen needs
to be present at the electrode inlet. During operation this can be achieved through
recirculation of the fuel electrode outlet stream, or by supplementing the steam feed
with hydrogen from an external source. Internal recirculation is applicable only when
the SOEC is actively generating hydrogen. During system start-up or hot standby, and
external hydrogen source is needed. This introduces an additional interface of explosive
gas to the SOC module or its feed arrangements.
[0012] In a large installation, it is beneficial to centralize as many functions as possible,
where practical, to minimize the number of components multiplying with each module.
In particular, this applies for functions that do not need close thermal coupling
with the high-temperature environment. This applies for air feed, steam generation
and post-processing of product gases. However, with common upstream and or downstream
systems it is nevertheless needed to facilitate module-specific or at least module-group
specific operating state to allow for module overhaul without interrupting all operations.
[0013] The post-processing of the fuel side product gas typically includes at least drying
and compression.
[0014] As the reactant streams of the electrodes in SOEC operation are not combined, it
is essential to assure that excessive pressure differences over the electrolyte are
avoided in all operating situations. All potential failure situations relating to
reactant flow and pressure control arrangements need to be taken into account. Particularly
in the case of external upstream or downstream equipment affecting the system internal
pressure levels, a module specific safety release mechanism can be required to safeguard
certain failure combinations.
Short description of the invention
[0015] The object of the present invention is to achieve an advanced solid oxide cell system
with improved flow conditions and with improved safety conditions. This is achieved
by a recirculated solid oxide electrolyzer cell system, a cell comprising a fuel side,
an oxygen rich side, and an electrolyte element between the fuel side and the oxygen
rich side. The system comprises at least one supersonic ejector configured for recirculating
a fraction of gas exhausted from the fuel side of each cell and for providing a desired
recirculation flow rate of recirculated flow, the ejector having at least one nozzle;
means for providing at least one primary feedstock fuel fluid to said nozzle of the
ejector, which nozzle has a convergent-divergent flow channel through which the fluid
will expand from an initial higher pressure to a lower pressure, wherein the ejector
and possible sources of leakage are contained within a structure conveying non-explosive
side reactant to form leakage and explosive safe structure, and the system comprises
a nested arrangement for at least one feed-in route and an exhaust route, the arrangement
being nested within a structure conveying non-explosive reactant, and a trim heater
arranged within the structures to provide heat to both fuel side and oxygen rich side
flows.
[0016] The focus of the invention is also a method of recirculated solid oxide electrolyzer
cells. In the method is supersonically ejected for recirculating a fraction of gas
exhausted from a fuel side of each cell and for providing a desired recirculation
flow rate of recirculated flow, is provided at least one primary feedstock fuel fluid
to a nozzle of an ejector to expand the fluid from an initial higher pressure to a
lower pressure, and in the method is contained the ejector and possible sources of
leakage within structures conveying non-explosive side reactant to form leakage and
explosive safe structure, and at least one of an feed-in route and an exhaust route
are located in a nested arrangement within a structure conveying non-explosive reactant,
and is provided heat to both fuel side and oxygen rich side flows within the structures.
[0017] The invention is based on use of a supersonic ejector configured for recirculating
a fraction of gas exhausted from the fuel side of each cell and for providing a desired
recirculation flow rate of recirculated flow. At least one primary feedstock fuel
fluid is provided to a nozzle of the ejector, which nozzle has a convergent-divergent
flow channel through which the fluid will expand from an initial higher pressure to
a lower pressure. The invention is further based on that the ejector and possible
sources of leakage are contained within a structure conveying non-explosive side reactant
to form leakage and explosive safe structure, and based on a nested arrangement for
at least one feed-in route and an exhaust route within a structure conveying non-explosive
reactant, and based on a trim heater arranged within the structures to provide heat
to both fuel side and oxygen rich side flows.
[0018] The benefit of the invention is that high recirculation rate is achieved, which means
small concentration gradients, more even thermal distribution and lower degradation.
Also, explosion risk can be eliminated according to the invention. Furthermore, the
invention allows for minimizing complexity, number of interfaces and potential sources
of leakage relating to SOC modules, allowing for application level cost savings.
Short description of figures
[0019]
- Figure 1
- presents a single cell structure.
- Figure 2
- presents an exemplary SOC system according to the present invention. In the figure
2 line which is drawn to cross over another line is not connected to said line.
- Figure 3
- presents an exemplary exemplary heat exchanger according to the present invention.
Detailed description of the invention
[0020] In embodiments according to the present invention is presented a recirculated solid
oxide electrolyzer cell system in which a cell comprises a fuel side 100, an oxygen
rich side 102, and an electrolyte element 104 between the fuel side and the oxygen
rich side as presented in figure 1. The exemplary system according to the invention
is presented in figure 2 and comprises at least one supersonic ejector 120 configured
for recirculating 109 a fraction of gas exhausted from the fuel side of each cell
and for providing a desired recirculation flow rate of recirculated flow. The ejector
has at least one nozzle 122. The system comprises means 124 for providing at least
one primary feedstock primary feedstock fluid to said nozzle of the ejector 120, which
nozzle has a convergent-divergent flow channel through which the fluid will expand
from an initial higher pressure to a lower pressure. Means 124 are for example a supply
line or a storage for a gas feed stock or supply line for a liquid feed stock and
means such as an evaporator for forming a gaseous fluid by evaporating the liquid
as well as necessary piping connecting parallel feed stock supplies to a common feed
in pipe connected to the ejector primary nozzle. Heat exchangers 105, 152 are located
both in the fuel side and in the oxygen rich side feed routes. In an exemplary embodiment
the system according to the present invention can comprise a heat exchanger 153 (Figs
2 and 3), which is connected to the supersonic ejector 120. Figure 3 presents the
flows which said heat exchanger takes in and releases out. The system location of
heat exchanger 153 is presented in figure 2. Product hydrogen and steam can flow (146,
Fig 2) from electrolysis as presented in figure 3. In low pressure heated steam can
flow to the electrolysis circulation 109, and steam can flow 124 into the heat exchanger
153. Product gas and residual steam can flow out through line 134. This arrangement
describes benefits of a double piping structure as combined to heat recovery in the
system.
[0021] The ejector 120 can block spikes in the inlet steam pressure from affecting the SOC
cells. The product gas outlet is beneficially coupled with a mechanical or electrochemical
compressor. The compressor shall regulate the SOC outlet pressure to a fixed value
or according to a setpoint provided by the SOC control system. A buffer volume and
flow restrictor elements may be needed in between depending on compressor and pressure
control dynamics. A passive pressure balancing arrangement can be used to throttle
one or more reactant outlet route(s) to maintain the differential pressure within
allowable bounds. A gas collection pipeline network, collecting the result gas from
multiple modules, can beneficially act as a buffer volume.
[0022] An air-recirculation arrangement, using a subsonic ejector to mix SOC outlet air
with inlet air can be used to further improve thermal self-sufficiency and cost of
air feed.
[0023] In system control the ejector can be beneficially dimensioned for a ~1:3 modulation
range without moving parts. The operation window can be extended by providing a bypass
line to feed a portion of primary feedstock past the ejector at high feed rates. The
system according to the present invention can comprise the primary feedstock fluid
and/or the bypass line 128 supplemented by a carbonaceous feedstock and/or other reducing
compound. The system can heat up on heated air and/or radiative heat elements. Protecting
stacks for re-oxidation by electronic fuel side protection can be performed by feeding
a small electrolysis current in voltage control mode near open cell voltage (OCV)
conditions. Additionally or alternatively, the feedstock steam feed can be supplemented
with a small amount of hydrogen or other reducing compound, readily available in e.g.
a refinery environment, at startup. The system is thermally self-balancing, converging
towards a temperature where the cell resistivity provides thermoneutrality with the
given current density. Particularly in steam-electrolysis, this mechanism can be beneficially
used to avoid a need for excess heating. In co-electrolysis, the system can be started
and brought to nominal operating temperatures in steam electrolysis mode, introducing
the carbonaceous feedstock as primary or bypass feedstock once the carbon-formation
temperature range has been avoided. Unlike the SOFC operating mode, the thermal management
of the SOC cells in SOEC mode is insensitive to the rate of air feed. This allows
for common-railing a large number of hot cores to a shared air feed and exhaust route
regardless of variations in operating point, including start-up and shutdown. Thus,
in one embodiment according to the present invention, the system can comprise means
130 for performing common-railing for hot cores to a shared air feed 132 and exhaust
route regardless of variations in operating point. In combination with common steam
supply and product gas extraction routes this allows for shared pressure balancing
equipment for all common-railed hot cores, further reducing cost in large installations.
This is a particular benefit in the case of pressurized SOEC modules.
[0024] In the recirculated solid oxide electrolyzer cell system according to the present
invention the ejector 120 and possible sources of leakage are contained within a structure
144 conveying non-explosive side reactant, such as e.g. air or steam gas, to form
leakage and explosive safe structure. The system can comprise a nested arrangement
for at least one feed-in route 124 and the exhaust route, which is nested within the
structure 144 conveying non-explosive reactant. Furthermore, the system according
to the present invention comprises a trim heater 148 to provide heat to both fuel
side 100 and oxygen rich side 102 flows. Heat exhangers and fluid conveying structures
involving high surface temperatures can all be placed within the structures 144 conveying
non-explosive reactant. Exemplary figure 2 presents that heat exchanger 105 can be
optionally 145 within the structures 144 or not.
[0025] The hot core, containing all high-temperature equipment but no moving parts, minimum/no
high temperature feedthroughs and minimum component count is an excellent building
block towards large systems. Cold air supply, steam supply, product gas post-processing
and the electrical power supply route can easily be centralized to megawatt-range
to serve multiple cores. The low or moderate temperature interfaces or the individual
hot cores are compact, inexpensive, and easy to handle, allowing for easy disconnection
and "hot" swapping of individual hot cores. The cost, complexity and component count
relating to the multiplying hardware (SOC stacks + dedicated balance of plant) in
a large-scale system is minimized, thus optimizing both capital and operational expenditure.
The benefits of the ejector-recirculated integrated hot core are apparent both in
the case of an atmospheric or pressurized operating point. The atmospheric operation
avoids complexity in the SOEC module structures and air supply, allowing to minimize
its cost. A pressurized system, in turn, provides apparent benefits in the product
gas pressurization. The cold and gas-tight outer shell of the symmetric integrated
hot core can easily be adapted to a pressure vessel shape capable of handling high
pressures. The high-temperature internal parts, where only small differential pressures
are present, can be maintained essentially unaffected by the pressurization.
[0026] The system according to the present invention can comprise a bypass line 128 for
extending operation window of the system by providing a portion of primary feedstock
past the ejector 120 at high feed rates. The bypass line can be 128 arranged co-axially
with the exhaust route 132 to contain explosive mixture inside a non-explosive supply.
3D-printing of ejector structures can be used to facilitate the desired alignment
of the flow routes. The system according to the present invention can comprise an
enclosed and internally insulated hot core to eliminate hot feedthroughs. The enclosed
and internally insulated hot core without hot feedthroughs eliminates the risk of
hot surfaces external to the module. The hot cores can be EX-classified to allow for
flexible placement e.g. in a refinery environment. The structure is beneficial for
both atmospheric and pressurized operation.
[0027] In an exemplary embodiment the system can also comprise a low temperature and gas-tight
outer shell of the hot core to be adapted to a pressure vessel shape capable of handling
high pressures. A pressure relief arrangement, providing quick pressure balancing
and/or releasing towards atmospheric or the air outlet line can be included to safeguard
possible pressure control abnormalities. The waste-gate can also be used during system
startup or shutdown conditions.
[0028] The system according to the present invention can comprise compressorturbine arrangement
142 of the air feed 132 controlled to avoid fast pressure changes. In such a case,
the pressure differential release mechanism can connect the fuel side and oxygen rich
side outlet-streams in case of pressure control anomalies.
[0029] In one embodiment according to the present invention cell stacks 103 can be arranged
so that part of cells operates in SOEC mode and another part of the cells operates
in SOFC mode. In a reversible (SOFC) mode, the steam supply is replaced by hydrogen
or preferably hydrocarbon supply, using same supply and preheating means as for SOEC
supply gas. The same ejector can provide the required recirculation in both modes.
To balance the difference in primary flows, a bypass is used in SOEC mode.
[0030] During system start-up, fuel feed can be supplemented by steam to maintain suitable
volume flow in the ejector primary and for oxygen-to-carbon ratio management. Alternatively,
air can be used as supplemental feedstock to facilitate catalytic partial oxidation
(CPOx) operation.
[0031] The above described supplemental steam feed line can also be supplemented with a
carbon containing, e.g. CO2 feedstock. This allows for operating the system in co-electrolysis
mode whilst still being able to start the system in pure steam electrolysis mode.
Thus, carbon formation issues relating to low temperatures at startup can be avoided.
To prevent carbon formation in the cooldown of the result gas, supplemental steam
may be injected to the result gas extraction line. An ejector structure can be used
for the injection, providing efficient mixing and allowing for boosting the pressure
to overcome pressure losses in downstream components, allowing for a more compact
design thereof.
[0032] The exemplary system according to the present invention can comprise an external
afterburner arranged in parallel with a product gas line 134 (Fig 2). Due to good
thermal self-sufficiency of the ejector-recirculated design, the afterburner can be
placed on the pressure relief line, i.e. outside of the hot core, thus avoiding hot
valve arrangements and additional equipment. The afterburner can be arranged in parallel
135 with the SOEC product gas pressurization stage by using e.g. a simple non-return
valve 156 (Fig 2) to redirect the flow when the compressing is stopped. Another non-return
valve 155 is located in the parallel product gas line 134.
[0033] The CO2 can be captured from the fuel side outlet stream upstream of the burner.
Alternatively to the burner, the residual H2 and CO can be recirculated back to the
system feed or recovered for other purposes. Thus, the system achieves zero carbon
emissions or negative net carbon emissions if biogas is used as source.
[0034] Alternatively, a passive non-return valve functionality can be implemented in the
hot environment to facilitate partial or full thermal burning in SOFC mode, whereas
in SOEC mode, the pressure is regulated such that the route is closed.
[0035] The SOC cells and stacks are preferably connected in electrical series comprising
hundreds of cells to allow for a cost-efficient connection to industrial DC or AC
distribution levels.
[0036] In preferred embodiments according to the present invention, the moduleinternal ejector
recirculated concept can be utilized to facilitate explosion-safe start-up hydrogen
supplement without need for an external hydrogen interface. By arranging a secondary
release route from the fuel side recirculation loop, it is possible to use steam as
entraining force in an ejector to entrain hydrogen rich mixture from the product gas
line. The secondary release route can also function as a safety mechanism to avoid
adverse pressure conditions at the solid oxide cells in case of pressure control abnormalities.
[0037] In an exemplary embodiment according to the present invention the system can comprise
a controllable secondary release route 134 from the fuel side to the ambient. This
release route functions as a safety mechanism in case of pressure control abnormalities.
During start-up, the release route is kept open while steam is supplied to the ejector
primary. The therethrough generated suction towards the hydrogen extraction line causes
reverse (inward) flow in said line, thus supplementing the steam in the recirculation
loop with a hydrogen rich mixture from the application side result of a gas distribution
network. The hydrogen extraction line may include a fixed or controllable flow restriction
element for pressure balancing during normal operation. The flow restriction in the
reverse flow configuration may be dimensioned to passively give rise to a nonexplosive
mixture of the entraining steam and the entrained hydrogen, thus allowing for a completely
passive maintaining of explosion safe conditions in the recirculation loop during
an emergency shutdown, given that external steam feed and the external hydrogen collection
networks remain operational. This eliminates not only the need for a module specific
startup hydrogen feed connection but also the need for active module specific shutdown
reoxidation protection means. In one exemplary embodiment the system can comprise
a supplemental ejector 154 driven by a steam feed to entrain hydrogen rich fluid in
reverse flow from the outlet gas interface. A preferable embodiment is to place this
ejector in series with the main fuel recirculation ejector 120. Thus, the supplemental
ejector entrains hydrogen-rich fluid to an intermediate pressure level, still sufficient
for driving the main ejector. In figure 2 is presented means 158, i.e. a valve, to
direct flow to the secondary release route 150. Same means can also be used for controlling
the pressure difference between fuel and oxygen side in normal operation. The means
158 can also be used to control the hydrogen fraction in the fuel side recirculation
when hydrogen is entrained from the product gas line through ejector 154, by recirculating
a portion of the fuel side outlet flow 134 to said ejector. Control valves 160, 164
can be used to enable the reverse flows in the secondary release route 150. With a
steam supply pressure of e.g. 3-3.5 bar(g), the intermediate pressure can be in the
range of 1-1.5 bar(g), whereby supercritical conditions can be obtained in both ejectors,
giving a very predictable performance. Particularly in the supplemental ejector, designed
for a fixed low entrainment in the range of 0.05-0.2, the mixing ratio of hydrogen
and steam becomes highly predictable and independent of small variations in the pressure
level on the suction side. In this embodiment, the secondary release route can be
arranged along the product gas line or the supplemental steam feed line, if present.
A benefit of both embodiments is that the provision of safety gas can be arranged
passively, eliminating the possibility of a single device failure to give rise to
a hazardous mixture. To prevent possible buildup of leaking hydrogen from the fuel
side in the conveying structures in the event of an air side supply disruption, an
additional steam driven ejector may be utilized to facilitate emergency ventilation
within the module. For example, an exhaust route from the top of the hot core module
may be arranged to pass through a highentrainment mode steam driven ejector to the
ambient. In normal operation, when the module is actively flushed by e.g. oxygen side
reactant, only a small amount of hydrogen from possible leakages is expected to accumulate
in the compartment top. Thus, the primary feed of the ejector can be kept off, whereby
a small exhaust flow, driven by the overpressure inside the module, is obtained. In
emergency mode, the primary steam feed can passively be activated whereby the outward
flow is amplified, thus increasing the capability to remove accumulated hydrogen.
Matching this flow with the amount of hydrogen entrained by the previously described
steam driven hydrogen ejector, it is possible to passively safeguard sufficient removal
of hydrogen even in the case of 100% leakage. As both the hydrogen feed and removal
rely on the same primary source of ene, i.e. steam, the combination has no dangerous
failure modes.
[0038] A further benefit of the arrangement is that explosive zones can be avoided within
and around the SOC modules. As a separate hydrogen feed line is avoided, explosive
species are only present in low pressure pipelines. A suitable pressure level for
the result gas collection line is e.g. 30 mbarg. Such low pressure limits the extent
of explosive mixture volumes that can arise around sources of leakage, whereby natural
ventilation can suffice for classifying the explosive areas as "negligible extent",
avoiding the need for costly explosion protection arrangements and equipment. This
also greatly simplifies provisions for making e.g. maintenance on individual modules,
when adjacent modules or pipelines remaining in operation do not cause an explosion
risk.
[0039] If e.g. in a refinery setting, the modules are to operate in an area which may contain
explosive gases, the invention is beneficial for obtaining an explosion-safe classification.
The low temperature of interfaces provide inherent protection against e.g. hydrogen
ignition temperatures and minimized amount of module specific equipment allows for
straight-forward explosion protection. With minimized amount of components external
to the hot module, ventilation requirements are also minimized. Centralized airfeed,
the supplied air being non-explosive, can be utilized to ventilate and overpressurize
compartments, whereby EX-classification of components as well as separate means to
monitor the presence of the ventilation can be omitted. Electrical equipment requiring
high ventilation can be placed external to the modules in a non-explosive environment,
or in enclosed overpressurized cabinets with liquid cooling.
[0040] In another exemplary embodiment according to the present invention the system can
be configured for reverse operation comprising means for fuel feed through at least
one of a primary feed route 124 and a supplemental feed route 128, i.e. a bypass line,
depending on the available pressure level. The means for fuel feed can be performed
e.g. by dimensioning the primary feed ejector 120 for a high-efficiency and high entrainment
ratio. Thus, this ejector can provide the required circulation in both operation modes
even without steam supplementing in SOFC mode. In case of a low pressure feedstock
such as biogas, the required recirculation is then obtained by feeding steam to the
primary ejector. The fuel side outlet gas in case of SOFC operation can be injected
either to the product gas line or to the secondary release route. The ejector-based
circulation allowing for eliminating thermal losses in the fuel side recirculation
facilitates good thermal self-sufficiency of the hot balance of plant, even in the
absence of an afterburner to aid pre-heating of reactants. Thus, fuel cell mode operation
can take place without need for supplemental heating. As the power conversion stages
have been dimensioned for high current densities in SOEC operation, they are inherently
capable of high current densities also in SOFC mode, thus able to bring the current
density high enough for thermal self-sufficiency even in beginning of life conditions.
The capability to supplement the fuel side recirculation with steam overcomes issues
relating carbon formation in the event of otherwise insufficient recirculation rates.
[0041] In the exemplary embodiment according the present invention, in order to further
minimize the complexity relating to multiplying SOC modules, it is beneficial to centralize
the supply of sweep air (oxygen rich side reactant) to the system. Pressurization,
filtering and possible drying are thus implemented in larger scale, allowing cost
savings and reliability improvements. At low temperatures, low pressure air supply
network for the SOC modules can be implemented at low cost. Furthermore, the centrally
provided air can also be used for compartment overpressurization, explosion safety
ventilation and cooling of e.g. automation equipment, thus eliminating the need for
module specific means and related failure detection safeguards to perform said functions.
[0042] In embodiments according to the present invention, in SOEC operation it is beneficial
to operate the cells with a uniform temperature profile, i.e. near thermoneutral conditions,
to maximize the utilization of the cell active area. This can be achieved by the relatively
high fuel side recirculation rate obtainable with the ejector based circulation. Moreover,
the circulation reduces the concentration gradients across the fuel side electrode,
promoting a more even distribution of current density across the cell. It is beneficial
to heat up the incoming reactants on both sides near to the operating conditions.
The ejector-based integration concept allows for arranging the SOC stacks in an essentially
symmetrical arrangement around a central axis, providing equal flow paths for the
reactant supply and thus even conditions to all cells. Furthermore, a heater element
or array of elements can beneficially be placed within the symmetry to provide heat
up of both fuel side and oxygen side reactants within the conveying structures. The
inlet area of the stacks can be used as heat exchange surface for transferring the
heat to the reactants. The metallic interconnect plates of stacks, typically extending
exterior to the active area of cells, together constitute a very large heat transfer
area to which heat can be effectively transferred through radiation from an e.g. electrical
heating element. The symmetric arrangement facilitates equal heat delivery to all
locations. Thus, the desired heat transfer can be obtained without dedicated heat
transfer structures. Preferably, to manage the temperatures of the stacks, a model
based approach with real time calculation of thermal equilibrium is beneficial as
physical temperature measurements can provide misleading indications in the arrangement.
[0043] A further optimization of system operations can be obtained by applying recirculation
also on the oxygen rich side. One or more subsonic ejectors can be used to recirculate
outlet reactant back to the oxygen rich side inlet, thus reducing the need of fresh
feed of oxygen reactant whilst reducing the oxygen concentration gradient between
cell inlet and outlet. If there is a need for pure oxygen, oxygen can be separated
from the oxygen-enriched outlet gas. Separation can be accomplished using e.g. a separation
membrane. The oxygen depleted, nitrogen rich retentate from the separation could then
be supplied back to the SOC system in mixture with air. The reduced oxygen concentration
of such a feed has a beneficial effect on the cell voltages. The nitrogen-rich stream
could also come from another source.
[0044] In embodiments according to the present invention, when operating the SOC system
in fuel cell mode, the outlet gas from the fuel side electrode can be routed to the
product gas distribution, where it is either combusted in an afterburner, or the carbon
dioxide is separated, whereby the residual hydrogen and carbon monoxide can be re-utilized
or circulated back to the system gas feed. As in the fuel cell mode, carbon dioxide
is formed on the fuel side electrode without mixing with the oxygen rich side reactant,
its concentration in fuel side outlet gas is high, making separation attractive. If
operating on a carbon-neutral fuel such as biogas, capturing the outlet carbon dioxide
yields a negative carbon footprint operation. The captured carbon dioxide can subsequently
be used as e.g. feedstock for the production of e-fuels or other synthetic hydrocarbons.
[0045] In the embodiments according to the present invention, the post-processing of the
fuel cell operating mode can be performed so that outlet gas can be accomplished either
along the product gas collection line or along the secondary release route. In both
cases, provided that all parallel connected modules are operating in the same direction,
these functions can be centralized.
[0046] As a conclusion it can be stated that several benefits can be obtained by the steam-driven
ejector recirculated SOC according to the present invention. High recirculation rate
facilitates small concentration gradients, more even thermal distribution and lower
degradation. In co-electrolysis, high recirculation rate reduces risk of carbon formation
through reduction of local concentration variance at the SOC outlets. Thermal management
and modulation range can be improved because there are no high-temperature feedthroughs
and no heat losses in hydrogen recirculation, and thus compact and symmetrical arrangement
can be achieved with a shape naturally well suited for placement in a pressure vessel.
Elimination of explosion risk can be achieved by inherent double-piping, leak flushing
within hot parts, avoidance of hot surfaces outside the enclosed system core, avoidance
of high-pressure explosive gas interfaces and use of centrally supplied clean air
for compartment venting and overpressurization. The steam inlet can be arranged coaxially
with product gas outlet to facilitate beneficial thermal transfer and extend the double
piping to the feeds.
[0047] Summarizing, the invention relates to:
- 1. Recirculated solid oxide electrolyzer cell system, a cell comprising a fuel side
(100), an oxygen rich side (102), and an electrolyte element (104) between the fuel
side and the oxygen rich side, characterized by, that the system comprises at least
one supersonic ejector (120) configured for recirculating (109) a fraction of gas
exhausted from the fuel side (100) of each cell and for providing a desired recirculation
flow rate of recirculated flow, the ejector having at least one nozzle (122); means
(124) for providing at least one primary feedstock fuel fluid to said nozzle of the
ejector (120), which nozzle has a convergent-divergent flow channel through which
the fluid will expand from an initial higher pressure to a lower pressure;
wherein the ejector (120) and possible sources of leakage are contained within structures
(144) conveying non-explosive reactant to form leakage and explosive safe structure,
and the system comprises a nested arrangement for at least one feed-in route (124)
and an exhaust route, the arrangement being nested within the structure (144) conveying
non-explosive reactant, and a trim heater (148) arranged within the structures (144)
to provide heat to both fuel side (100) and oxygen rich side (102) flows.
- 2. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system comprises a controllable secondary release route (150) from the
fuel side to the ambient.
- 3. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system comprises a supplemental ejector (154) driven by a steam feed
to entrain hydrogen rich fluid in reverse flow from the outlet gas interface.
- 4. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system is configured for reverse operation comprising means for fuel
feed through at least one of a primary feed route (124) and a supplemental feed route
(128) depending on the available pressure level.
- 5. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system comprises means (130) for performing common-railing for hot cores
to a shared air feed (132) and exhaust route regardless of variations in operating
point.
- 6. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system comprises an enclosed and internally insulated hot core to eliminate
hot feedthroughs.
- 7. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system comprises a low temperature and gas-tight outer shell of a hot
core to be adapted to a pressure vessel shape capable of handling high pressures.
- 8. Recirculated solid oxide electrolyzer cell system in accordance with (1), characterized by, that the system comprises cell stacks (103) arranged to a part of cells capable
of operating in SOEC mode and arranged to another part of cells capable of operating
in SOFC mode.
- 9. Recirculated reversible solid oxide cell system in accordance with (1), characterized by, that the system comprises an external afterburner arranged in parallel (135)
with a product gas line (134).
- 10. A method of recirculated solid oxide electrolyzer cells, characterized by, that in the method is supersonically ejected for recirculating (109) a fraction
of gas exhausted from a fuel side (100) of each cell and for providing a desired recirculation
flow rate of recirculated flow, is provided at least one primary feedstock fuel fluid
to a nozzle of an ejector (120) to expand the fluid from an initial higher pressure
to a lower pressure, and in the method is contained the ejector (120) and possible
sources of leakage within structures (144) conveying non-explosive reactant to form
leakage and explosive safe structure, and at least one of an feed-in route (124) and
an exhaust route (132) are located in a nested arrangement within a structure (144)
conveying nonexplosive reactant, and is provided heat to both fuel side and oxygen
rich side flows within the structures (144).
- 11.A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is controlled reactants in a secondary release route from
the fuel side to the ambient.
- 12. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is driven by a steam feed to entrain hydrogen rich fluid in
reverse flow from the outlet gas interface.
- 13. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is operated inversely cell system operation by feeding fuel
feed through at least one of a primary feed route (124) and a supplemental feed route
(128) depending on the available pressure level.
- 14. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is performed common-railing for hot cores to a shared air
feed (132) and exhaust route regardless of variations in operating point.
- 15. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is enclosed and internally insulated a hot core to eliminate
hot feedthroughs.
- 16. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is adapted a low temperature and gas-tight outer shell of
a hot core to a pressure vessel shape capable of handling high pressures.
- 17. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method is arranged cell stacks (103) to a part of cells which are
operated in SOEC mode and to another part of cells which are operated in SOFC mode.
- 18. A method of recirculated solid oxide electrolyzer cells in accordance with (10),
characterized by, that in the method an external afterburner is arranged in parallel (135) with
a product gas line (134).
1. Recirculated solid oxide electrolyzer cell system, a cell comprising a fuel side (100),
an oxygen rich side (102), and an electrolyte element (104) between the fuel side
and the oxygen rich side, characterized by, that the system comprises at least one supersonic ejector (120) configured for recirculating
(109) a fraction of gas exhausted from the fuel side (100) of each cell and for providing
a desired recirculation flow rate of recirculated flow, the ejector having at least
one nozzle (122); means (124) for providing at least one primary feedstock fluid to
said nozzle of the ejector (120), which nozzle has a convergent-divergent flow channel
through which the fluid will expand from an initial higher pressure to a lower pressure;
wherein the ejector (120) and possible sources of leakage are contained within structures
(144) conveying non-explosive reactant to form leakage and explosive safe structure,
and the system comprises a nested arrangement for at least one feed-in route (124)
and an exhaust route (133), the arrangement being nested within the structure (144)
conveying non-explosive reactant, and a trim heater (148) arranged within the structures
(144) to provide heat to both fuel side (100) and oxygen rich side (102) flows, and
the system further comprising a bypass line (128) for extending operation window of
the system by providing a portion of primary feedstock past the ejector (120) at high
feed rates.
2. Recirculated solid oxide electrolyzer cell system in accordance with claim 1, characterized by, that the bypass line (128) is being arranged co-axially with an exhaust route (133)
to contain explosive mixture inside a non-explosive supply.
3. Recirculated solid oxide electrolyzer cell system in accordance with claim 1, characterized by, that the system comprises a controllable secondary release route (150) from the
fuel side to the ambient.
4. Recirculated solid oxide electrolyzer cell system in accordance with claim 1, characterized by, that the system comprises means (130) for performing common-railing for hot cores
to a shared air feed (132) and exhaust route regardless of variations in operating
point.
5. Recirculated solid oxide electrolyzer cell system in accordance with claim 1, characterized by, that the system comprises an enclosed and internally insulated hot core to eliminate
hot feedthroughs.
6. Recirculated solid oxide electrolyzer cell system in accordance with claim 1, characterized by, that the system comprises a low temperature and gas-tight outer shell of a hot core
to be adapted to a pressure vessel shape capable of handling high pressures.
7. Recirculated solid oxide electrolyzer cell system in accordance with claim 1, characterized by, that the system comprises cell stacks (103) arranged to a part of cells capable
of operating in SOEC mode and arranged to another part of cells capable of operating
in SOFC mode.
8. Recirculated reversible solid oxide cell system in accordance with claim 1, characterized by, that the system comprises an external afterburner arranged in parallel (135) with
a product gas line (134).
9. A method of recirculated solid oxide electrolyzer cells,
characterized by, that in the method:
is contained the ejector (120), at least one of an feed-in route (124) and an exhaust
route (133) and other possible sources of leakage within structures (144) conveying
non-explosive reactant to dilute leakages to explosion-safe levels,
- entrainment of a fraction of gas exhausted from a fuel side of each cell is performed
using a supersonic ejector for providing a desired recirculation flow rate of recirculated
flow
- is provided at least one primary feedstock fluid to a nozzle of an ejector (120)
to expand the fluid from an initial higher pressure to a lower pressure, is provided
heat to both fuel side and oxygen rich side flows within the structures (144), and
in the method is extended the operation window of the system by bypassing a portion
of the primary feedstock past the ejector 120 at high feed rates.
10. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the methodis performed the bypassing co-axially with the exhaust route (133)
to contain explosive mixture inside a non-explosive supply.
11. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the method is controlled reactants in a secondary release route from the
fuel side to the ambient.
12. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the method is performed common-railing for hot cores to a shared air feed
(132) and exhaust route regardless of variations in operating point.
13. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the method is enclosed and internally insulated a hot core to eliminate
hot feedthroughs.
14. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the method is adapted a low temperature and gas-tight outer shell of a hot
core to a pressure vessel shape capable of handling high pressures.
15. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the method is arranged cell stacks (103) to a part of cells which are operated
in SOEC mode and to another part of cells which are operated in SOFC mode.
16. A method of recirculated solid oxide electrolyzer cells in accordance with claim 9,
characterized by, that in the method an external afterburner is arranged in parallel (135) with a
product gas line (134).